Sulphite pulping process



June 4, 1963 G. H. ToMLlNsoN l1 `3,092,535

SULPHITE PULPING PRocE Filed April 27, 1960 2 Sheets-Sheet 1 INVENToR.

BY George H. Tom|nson,I[

ATTORNEY June 4, 1963 G. H. roML.1NsoN n SULPHITE PULPING PROCESS 2 Sheets-Sheet 2 Filed April 27, 1960 INVEN TOR.

1 2 TIME FROM START OF LIQUOR IMPREGNATION PERIOD HOURS George H. Tomlinson, II

REGULAR MAGNEFITE CooK Two STAGE COOK AFTER INJECTION OF 3.0 PERCENT MAGNEslA ATTO RNEY ttes The present -invention relates to Vthe manufacture of chemical pulps by a sulphite cooking process employing a relatively pure magnesium base cooking liquor having a relatively high total sulphur dioxide and essentially -free of excess sulphur dioxide, with an initial pH value in the range of 2.5-5.0. The process involves impregnation of the wood chips under a substantial superatmospheric hydrostatic pressure, draining of surplus cooking liquor until a desired liquor-wood ratio is present, rapid heating of the chips to a high cooking temperature in the range of R-200 C., and regulating the discharge of SO2 formed from sulphurous acid generated during the cooking period to maintain the pH value of the cooking liquor in the range of 3 .tl-4.0, as lmeasured at room temperature, during the period in which the major sulphonation of the lignin and dissolution ofthe wood occurs.

Such a process is disclosed and claimed in my prior joint application with George H. Tomlinson, Serial No. 625,164, filed Nov. 29, 1956, now U.S. Patent No. 3,046,182. This process, known in the industry as the Magneiite process, allows the pulping to proceed in less time and at considerably higher temperatures than `formerly employed in the production of sulphite paper pulps and, by various modilicatious, allows the production of relatively bright unbleached chemical pulps lfrom a wide variety of hard and soft woods or other cellulosic materials suitable -for the Whole range or paper making and chemical conversion. The pulps produced by this method are obtained in higher yields and have higher strength values than those obtained by the conventional acid sulphite process at a comparable degree of deligniiication.

The pulp residual liquor from this cooking method is of a character especially suitable for eiiicient recovery of its heat and chemical values in a cyclic recovery process, in which the residual liquor is separated yfrom the pulp, concentrated to a solids concentration of 50-70%, and the concentrated liquor burned in a chemical recovery unit with the recovery of heat, magnesium oxide and sulphur dioxide. The ash obtained consists essentially of magnesium oxide, while the sulphur content is obtained in the form of sulphur dioxide at a concentration of about 1% in the gaseous products of combustion. When this ash is separated from the gas, suspended in Water, .and the resultant suspension contacted with the combustion gases, the sulphur dioxide is absorbed to form a liquor consisting of magnesium bisulphite, Mg(HSO3)2, together with a minor proportion or magnesium monosulphite, MgSO3. rl`he type of cooking liquor desired for the described cooking process has but neglieible SO2 vapor pressure, which greatly simplies its processing. Furthermore, the stripping of a large quantity of SO2 from the digester .at the end of the cook, which ordinarily involves from o-ne to two hours of digester time, is no longer required, and iinally, the present cooking liquor can be stored at elevated temperatures up to 100 C. in open tanks instead of in the pressure accumulators normally employed for conventional acid sulphite liquor. Moreover, when an accumulator designed for 40 p.s.i.g. operating pressure is used, it is possible to store the fresh cooking liquor at 140 C. instead of at 65 C., which would be the maximum temperature that could `be used in such a vessel with conventional acid sulphite liquor. A further 'advantage of this process is that, due to the elimination of 3,092,535 Patented June 4, 1963 2 the handling of the large volumes of concentrated sulphur dioxide gas involved in the conventional acid sulphite process, the risk of atmospheric pollution from gas escape does not exist.

At the time of tiling said prior joint application, it was considered essential, in making conventional sulphite pulp, that the cooking liquor initially have a free SO2 considerably greater than the combined SO2, that is, that a large excess of sulphurous acid be present, `and that a slow temperature rise and a relatively low maximum cooking temperature be used. For example, sulphurous acid 40G-500%, or even more, in excess of the combined SO2, a temperature rise to C. in .about two hours, and a maximum cooking temperature of C. or less, were normally considered to be essential. In said prior joint application it was disclosed that, starting with a magnesium bisulphite liquor either free of or containing but a small amount of excess sulphurous acid and a pH of 2.5-5.0, a very rapid rise to much higher temperatures, i.e. temperatures materially above 150 C. and in the range of l60-200 C. may be advantageously employed. Small amounts of sulphurous .acid result from reaction between the magnesium bisulphite and the acids formed from the wood `during the digestion and by controlling the relationship between the pressure in the digester and its temperature, it is possible to control the quantity of excess sulphurous acid present during the cook. Thus if the pressure is not allowed to build up above the vapor pressure of steam, sulphur dioxide will be stripped off as formed, and cooled samples of the liquor taken during the cook will have a pH of 4.5 to 5.0. Whereas, the Wood can be pulped under these conditions it is preferred to allow the pressure to build up slightly above (eg. t5-15 psi.) that which would be established by the `water vapor pressure alone. Thus when cooking at 166 C. a pressure of 96 p.s.i.g. to 100 p.s.i.g. is used instead of 90 p.s.i.g. which is the pressure of saturated steam at that temperature. Under these conditions the pH of a sample of the cooking liquor when withdrawn during the cook and cooled to room temperature, will have a pH of 3.0 to 4.0, this `being the pH condition normally established and maintained during the cook. lt is therefore possible to start with a liquor having an initial pH of 4.0 to 5.0 on the one hand or of 2.5 to 3.0 on the other and still maintain a pH of 3.0 to 4.0 (cooled sample) during the cook, this pH `depending upon the extent to which the SO2 is retained or relieved from the digester.

In carrying out the `desired rapid rise to temperatures of the order of -200 C., this operation is facilitated by withdrawing a considerable portion, or possibly even all, of the surplus cooking liquor surrounding the wood chips following their impregnation. By this procedure low liquor to wood ratios of the order of 1.5 :l to 2.5:1 can be successfully employed owing to the fact that the character of the cooking liquor allows the use of solutions of a concentration suiciently great that the relatively small volume of liquor that is retained in the chips is adequate for the pulping reaction. At the same time the magnesium bisulphite reagent is sufficiently mild in its action on cellulose that the increased concentration in the liquor required at this low liquor to wood ratio does not adversely affect strength or yield of pulp, and under certain conditions it may actually result in increased yield and greater strength. This low liquor to wood ratio when employed, results in a reduced steam requirement for digestion, and also, because of the high solids content of the residual liquor under these conditions, a low steam requirement for subsequent evaporation of the residual liquor.

More particularly, the process disclosed in said prior joint application when used for the production of pulp from wood chips, comprises first displacing the air from pulp, with freedom from major odor problems.

the wood chips in the digester and then impregnating the chips with the new type of magnesium bisulphite cooking liquor. The impregnated chips are then heated to a predetermined temperature in the range of 160-200" C. This Velevated temperature is held for a predetermined period of time depending upon the temperature and pressure employed. At 160 C. this may be of the order of 3 to 4 hours, and at 190 C. may be as short as l() to 20 minutes, when the digester pressure is maintained about 5-15 p.s.i. above the vapor pressure of pure water at the digestion temperature, to give a chemical pulp having the desired quality and yield. The digester pressure may then be lowered to atmospheric, and the lliquor washed from the pulp either before or after the pulp is removed from the digester. The residual liquor thus obtained is evaporated to a solids concentration suitable for self-sustaining combustion and burned, a major portion of the liberated heat being converted to steam. The magnesium oxide and sulphur dioxide formed on combustion of the liquor are subsequently recombined in aqueous solution to form fresh cooking liquor of the desired composition. rl`he make-up for losses in chemical is added either in the form of magnesium sulphate to the residual liquor before its combustion or as magnesium oxide to the fresh cooking liquor.

The process disclosed in said prior joint application combines the kraft pulping process versatility with regard to usable wood species with the magnesia base recovery process disclosed in Tomlinson U.S. Patent No. 2,385,955 to produce a high yield strong relatively bright unbleached The magnesium base sulphite pulp, although materially stronger than regular sulphite pulp, was found to be somewhat weaker than kraft pulp.

The main object of the present invention is the provision of an improved magnesium -base sulphite pulping process of the general character disclosed in said prior joint application which is capable of producing a sulphite chemical pulp materially stronger than regular sulphite pulp and approximately as strong as kraft pulp produced from similar wood chips. More particularly, the invention is concerned with a two-stage relatively pure magnesium base sulphite pulping process of the character described in which the first cooking stage follows the disclosure in said prior joint application as to pH value, pressure and temperature of the cookingliquor, and whenV about 30-45% of the Wood weight has been dissolved, continuously or intermittently injecting an aqueous suspension of magnesium hydroxide, Mg(OH)2, into the liquor while being circulated in the digester, to substantially increase the pH value of the liquor to a maximum value slightly on the acid side of neutrality, i.e. in the range of 6.0-6.5 pH, as measured at room temperature, and maintaining these conditions substantially throughout the second cooking stage. The resulting pulp has been found to be substantially stronger than when the cooking is completed with the cooking liquor at a pH in the range of 3.0 to 4.0.

IThe various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the acompanying drawings and descriptive matter in which a preferred embodiment of the invention is disclosed.

In the drawings:

FIG. 1 is a schematic illustration of a pulp mill digester and chemical recovery system; and Y FIG. 2 is a curve sheet showingrcomparative mag nesium base sulphite pulp cooking processes.

`The present process can be carried out in a conventional sulphite pulp mill and like the cooking process in 4 said prior joint application involves the following operating stages:

(1) The digester 10 is substantially filled with wood chips, which may be any one of a wide variety of hardwoods or softwoods, or a mixture of several such Woods, in the usual manner and treated to promote subsequent penetration of the cooking liquor, for instance by presteaming at atmospheric pressure to displace air.

(2) The magnesium bisulphite cooking liquor, prepared as hereinafter described, and at a temperature about C. is then added until the digester is completely filled. In the example indicated in FIG. 2, the liquor initially has a total SO2 content of about 4.0%, a combined SO2 content of 2.0% and a pH of 3.5. The free SO2 will thus be 2.0% and there will be no excess sulphurous acid present. The terms total SO2, free SO2, and combined SO2 are used herein as currently defined by the Technical Section, Canadian Pulp and Paper Association (Data Sheet C-OOc, lune 1955). The pumping is continued until a digester hydrostatic pressure of about 90 p.s.i.g. is attained, to aid penetration of the chips.

(3) After about 30 minutes impregnation under these conditions a quantity of cooking liquor is withdrawn to give a iinal liquor to ydry Wood ratio of about 4.5 to 1.0. The digester pressure is dropped to atmospheric. The drained liquor is passed to the storage tank from which it Was initially withdrawn, and retained for use in the next digestion.

(4) After withdrawal of the excess cooking liquor, the contents of the digester 10 `are indirectly heated by circulation of a stream of liquor through an associated heat exchanger 12, in such a manner as to rapidly reach an elevated temperature in the 1GO-200 C. range, c g. 166 C., in about 35 minutes where it is held for a predetermined period, e.g. 11/2 hours, the digester pressure meanwhile being allowed to increase as the digester temperature lrisesand advantageously maintained at a value slightly greater, e.g. 5-15 p.s.i., than that .for the vapor pressure of pure water at the digester temperature which pressure would, for example, be 90 p.s.i.g. at 166 C.

(5) As the digester temperature and pressure are increased as described, the pH of the cooking `liquor tends to rise due to the transfer of SO2 from the liquid phase to the gas phase in the digester. In the example shown, the pH increased from a value of 3.5 to a value of 4.25. Where more efficient chip packing is obtained, the volume of gas space in relation to the remaining liquor would be from the digester. The total SO2 and the combined SO2 values also drop off rapidly as the SO2 is consumed in chemical reactions in the digesting process, as shown in solid lines in FlG. 2.

In ordinary bisulphite pulp cooking, the bisulphite tends to become exhausted after a predetermined period, forming rst thiosulphate and later elemental sulphur. Once any thiosulphate is present in a bisulphite solution, more thiosulphate is rapidly formed in a chain reaction dependent on the concentration of bisulphite and the thiosulphate at any given time. It is important that adequate Ibisulphite be initially present to complete the cook. The wood chips must be irst well impregnated so that the bisulphite is immediately available for the necessary sulphonation of the lignin. No substantial surplus of bisulphite over that actually required for cooking should be available as it would otherwise accelerate the undesirable thiosulphateforming chain reaction.

In accordance with the present invention, the later portion of the cooking process is formed by a second cooking stage in which the pH is increased to a Amaximum value slightly on the acid side of neutrality. This has been found to give a stronger pulp in essentially the same yield, and for any degree of delignication, than -that obtained when the digestion is completed in the 3 to 4 pH range by the method described in said prior joint application. This may be contrasted with bisulphite cooks using a soda xbase where pulping is completed in a second stage at alkaline pH and where such treatment results in considerable loss in yield compared with single stage bisulphite pulping. This is accomplished by adding magnesium hydroxide, Mg(OH)2, to the digester after `about 30- 4.5% of the wood Weight has been dissolved. Preferably a suspension of magnesium hydroxide is injected into the cooking liquor while it is being circulated. This reacts with any excess sulphurous acid present to produce bisulphite and then with a portion of this bisulphite to produce magnesium sulphite. It simultaneously Vreacts with the acetic acid present to form magnesium acetate, thus increasing the pH as measured on a cooled sample withdrawn from the digester to a maximum value of about 6.5. The digestion is completed under these conditions.

The process set forth in said prior joint application operates with a cooking acid at a concentration of about 4% total SO2 and at a pH of about 2.5 to 5.0, this latter pH being essentially the upper limit because of the low solubility of magnesium sulphite which forms at high pH and the essentially complete insolubility of magnesium oxide and magnesium hydroxide. The solubility of magnesium sulphite is 0.615% as magnesium sulphite or 0.38% as sulphur dioxide at 95 C., While that of magnesium hydroxide is only 0.000042% as magnesium hydroxide at 100 C. Thus it is impossible to impregnate the chips with sutiicient magnesium hydroxide and/or sulphite to carry out an alkaline cook. Since chip penetration in the case of soft woods takes place through fiber cavities or lumens which are only of the order of 20 microns in diameter, transferring from lumen to lumen through the so-called bordered pits which have a diameter of the order of 5 microns, it is apparent that the digestion reagent must be in solution to obtain penetration.

From the results obtained it appears that during the early stages of the digestion the ber component of the wood is protected from degradation insofar as its ultimate physical characteristics are concerned, and advantage may therefore be taken of the relatively rapid reactions which leadto deligniiication which occurs in the pH range of 3 to 4. This in turn allows the use of magnesium bisulphite which has ample solubility at this pH to supply adequate dissolved reactant for the Wood. It has now been established that following this initial bisulphite stage the :further dissolution of the sulphonated lignin can be carried out in a reasonable time at the higher pH obtainable by the addition of magnesium hydroxide, lMg(OI-I)2, andthat this second stage treatment results in the production of a stronger pulp, presumably because the fiber is subject to less attack, relative to the extent of the delignication simultaneously obtained, than when the digestion is concluded at the lower pH. Moreover it has been established that this second stage treatment with magnesium hydroxide does not result in additional dissolution of cellulose or hemicellulose which would result in a lower yield.

The quantity of magnesium hydroxide added for the second stage cooking Iis preferably between 0.5 lb. and 4.0 lbs. (as magnesium oxide) per 100 lbs. of Wood chips. Quantities in excess of 4.0% of the wood quantity should not be used since they will result in deposition of magnesium sulphite crystals in the digester and Within kthe pulp. The Mg(OH)2 addition should be made slowly to allow equilibrium between the higher pH liquor Vdiffusing into the chips and the more acidic liquor diffusing out. The dclignification reactions proceed more slowly as a result of the increased pI-l .obtained after injection and therefore a longer cooking time, land/ or a higher temperature should be used to complete the digestion.

As shown lin FIG. 2, representative digester conditions for the standard Magnete magnesium bisulphite cooking process as disclosed in said prior joint application -are shown in vsolid lines for aA digestion period of approximately four hours, at the yend of which period the total SO2 and combined SO2 Values are practically gone. In accord-ance with the invention la magnesium hydroxide suspension, or slurry, equivalent to 3.0% MgO based on the weight of the Wood, is injected into the digester liquid, While 'the cook is still in the 5 5-70% yield range, and preferably directly into the cooking liquor circulating line, as indicated in FIG. l. The pH of the liquor is rapidly increased to about 6.0 and maintained at that value for the remaining part of the cook. To compensate for the slower rate of reaction at the higher pH, particularly for softwood cooks, the cooking liquor temperature was increased to about 175 C. and the pressure allowed to rise to about 130 p.s.i.g. Digestion was allowed to proceed for the same length of time and as indicated in broken lines, the total SO2 .and combined SO2 values vwere substantially higher at the end ofthe cook than in standard Magnete magnesium bisulpthite cooking.

The possibility of using this second slightlyacid stage vwith magnesia Ibase depends on the fortunate balance Vcal to carry out the complete delignification, from` .the

start .of the digestion, at a pH in the range of 6.0 to. 6.5.

The examples shown in Table I indicate .digeste-r conditions lfound satisfactory: for carrying out this procedure for spruce and elm Woods, and the properties of the resuiting pulp. A standard Magnete cook is shown in each case for comparison.

TABLE I Spruce Elm Stand- Two- Stand- Twoard Stage ard Stage Cook 3583 3575 3579 #3527 Initial stage:

Time at 166 C hrs 3 1,1/1 3 1% Increased pH stage:

Initial pH (cooled sample) 6. 3 6. 3 Time hrs 2 2% Temperature C 170 166 Filial Pulp:

Roa chlorine number 7v 4 8.0 5. 4 6. 6

Yield:

Accepted 52. O 53. 4 49. 5 49. 4 ReJeets 0.5 V0. 8 1, 5 1. t)

Total 52. .5 54. 2 51.0 51. 3

Strength Tests at 450 cc. Canadian Frecness:

Tappi Burst 69 95 39 53 Tappi Tear 94 74 89 Bln-st 1/2 Tear 114 142 76 97 Tensile, Breaking length, metres 12. 4 13. 8 7.9 9. 4

It can be seen from these examples that the addition of magnesium hydroxide for a second stage increased pH cook results in a material increase in pulp strength. The TAPPI tear value yof 89 for the elm is particularly satisfactory in view of the short liber length characteristics of this species. On bleaching in a four stage sequence (5% C12; 2% NaOH; 2% C102 as- C12; 0.5% hypochlorite) the yTAPPI tear test actually increased to and a brightness of the bleached pulp of 93.9 G.E. was obtained. It is thus apparent that pui-ps produced in accordance With this invention are obtained in high yield, are very strong and can be easily bleached to very high brightness levels.

-bleaches are of a similar nature.

Vln another series of experiments using spruce the burst plus halt tear values as obtained from standard hand sheets after 50 beating time in a Standard Valley lBeater were as follows:

Test results have shown that the two-stage magnesium base pulping process of this invention produces pulp which is very readily bleached similar -to the standard Magneiite pulp of said prior joint application. The recults `are summarized in Table Il and show that in Athe hardwood pulp, the amount of chlorine used relative to the chlorine demand is about the same as used for sulphate pulp, but the two-stage pulps described bleach to much higher brightness, and the bleached pnl-ps show a lower reversion and retain their viscosity better. Further the two-stage pulps chlorinate much more readily and a higher proportion of the bleach is used -as chlorine, lowering the overall cost. The results of the softwood The kraft pulp could not be bleached to high brightness using the four-stage bleach procedure, and a sodium hypochlorite prebleach had to be used. The two-stage pulps on the other hand, were readily chlorinated to a low permanganate number and bleached to 90 G.E. brightness or higher in the tourv stage procedure.

TABLE 1l Comparzson of Bleach Requzremcnts Hardwood softwood Cook Cook Cook Com- No. No. N o. 3574 gmc@ 3559 e577 pruccsul hate T o' Pine T o' TWO' L p Stage Krait Stage Stage Elm Spruce Pine Chlorine No 4. 4 3. 6 7. 4 6.9 11.1 Unblcached Viscosity-- 21. 4 23. 7 27.1 24. 2 27. 2 Hypo prebleach (percent as chlorine) 4. 3.0 3.0 3.0 6.0 10.0 Caustic (percent) 2. 0 2. 0 2.0 2.0 2.0 Chlorine dioxide (percent as chlorine) 3. 3 2.0 3.3 2.0 2.6

Sodium hypochlorite Y (percent as cnloriue) 0. 5 0.5 1.0 0. 5 0.5 Total bleach (as ohlorino 6. S 5. 5 11. 3 S. 5 13. 1 Y Final Viscosity 12.4 17.4 14.6 20.7 22.6 Final Brightness- 89. 6 94.8 88.2 89. 6 93. 3 Points reverted 2. 6` 1.8 2.6 2.4 2.3

The described magnesium bisulphite pulpy cooking process is especially adapted to form part of a relatively simple and inexpensive cyclic system in which available heat and chemicals are effectively recovered from the digester residual liquor and the recovered chemicals recombined to form fresh cooking liquor for repeating the cycle. As schematically illustrated in FIG. l, the described cooking process is carried out in the digester 10, to which the wood chips and cooking acid are supplied in a well known manner. The cooking liquor is circulated through and indi- -rectly heated in an external heat exchanger 12 by a pump 14. A regulable supply line 16 for the magnesium hydroxide suspension is connected to the inlet side of the Ypump 14. This permits adjustment of the pH ofthe pletion of the two-stage cook through a blow tank 20 to the washers 18, from which the weak residual liquor is digester.

272 to a solids concentration ot 5ft-70%. The concentrated liquor is sprayed into the upper end of a chemical 'recovery unit 24 along with combustion air and burned in suspension under self-sustaining combustion conditions in the furnace section thereof. The fresh combustion gases with the M-gO ash particles in suspension and con- 'taining released SO2 are passed through the associated 'steam generating section 2.6 and air heater and economizer '28. The suspended ash is separated in multi-cyclones 30 and passed to a retention tank 32, vacuum filter 34, and slaking tanks 36. Make-up MgO is added to the system vat the tanks 36, mixed with water to form a suspension of Mg(OH)2, and delivered in part to the supply line 16.

In the normal Magnete cook, as described in said prior joint application, the acids formed from the wood as the digestion proceeds react with bisulphite to form sulphurous acid. In order to prevent the digestion liquor from becoming too acid, the digester is allowed to relieve. Thus the sulphurous acid formed from the bisulphite gives off sulphur dioxide which is carried with water Vapor from the This sulphu-r dioxide may be relatively easily recovered by absorption in fresh cooking liquor after condensation of the Water vapor. However, additional sulphur dioxide is lost during the blow and the washing rstage where it is diluted with air and not readily recovered. As disclosed in said prior joint application, magnesium oxide or hydroxide may beadded to the liquor following separation from the pulp at the washer to reduce Ythe disengagement of SO2 from solution during evaporation of the liquor. By the present invention independent 'loss of SO2 from solution is prevented not only during the iinal stage of the cook, where it may be readily recovered but also in the blow and washing stages where,

because of dilution with air, economic recovery is extremely difficult. Experience in an acid sulphite magnesio. base mill, where the separated liquor following pulp washing is neutralized prior to evaporation, has established a make-up requirement of 50 lbs. of sulphur and 20 lbs. of magnesia per ton of pulp. By employing the procedure disclosed, it should be possible to approach the theoretical sulphur loss of only 32 lbs. sulphur per ton of pulp which would result from a magnesia loss of 20 lbs. of SO2 were not lost to the atmosphere as is the case with previous methods. "Ihus even in cases where the extra pulp strength might be of no special advantage, neutralization in the digester will provide special recovery benefits in contrast to neutralization of the separated liquor.

In addition a partial neutralization of the liquor during the actual digestion has the .benecial elect of holding extra combined SO2 in the digester towards the end of the cook where it has been largely depleted from reaction with the lignin and from side reactions such as formation of thiosulphate. Holding this SO2 in the digester, which would otherwise be removed from it insures an adequate supply of reagent to carry the digestion to the desired degree of delignilication.

The gaseous products of combustion and released SO2 are discharged by the induced draft fan 38 to a packed gas cooling tower 40, in which the ascending gas is contacted with a descending flow of recycled cooled liquor to which a portion of the Mg(OH)2 slurry from tank 36 is added, which results in a partial absorption ot SO2 from the gas. The gases leaving the upper end of the cooling tower 40 pass through Ya Venturi gas scrubber 42, collec tion tank 44, a second Venturi gas scrubber 46, and nally through a cyclone separator 48 to the stack. In both the Venturi scru-bbers 4Z and 46, the gases are contacted with a spray of recycled liquor, to which controlled amounts of Mg(OH)2 have been added, for absorption of additional SO2 from the gases. The liquor droplets in suspension are separated from the gas stream by the change of direction and low velocity in the tanks 44 and 48. Liquor from the primary absorption system leaves collection tank 44 and goes to acid storage tank 5S where the liquor is recycled through a Venturi scrubber 56.

Normally the liquor leaving the primary absorption system will contain magnesium bisulphite and a small amount of magnesium monosulphite. `By adding a portion 'of the makeup sulphur dioxide coming from the sulphur burner 52 and gas cooler 54 to the Venturi 56, the composition of the l-iquor recycling from tank '58 may be controlled to give theiinal adjustment of liquor composition, i.e. Ito provide a solution having a chemical composition corresponding approximately to the formula Mg (H503 s The additionalmake-up SOzrequired to keep the system in balance, goes directly Ito the duct carrying the combustion gases to the cooling tower 40 and mixes therewith. The primary purpose of the cooling tower 40 is to cool the hot furnace ue gas to a temperature of about 40 C. which is suitable for SO2 recovery in the Venturi system. Water vapor is-condensed from the vgas at -this point rather than in the first Venturi 42 Where it would dilute the product acid. The liquor produced in the cooling ltower is run to -the recycling liquor for second Venturi 46. The controlled addition of Mg(OH)2 at the second Venturi gives a liquor having a pH of 5.5 to 6.0 which results in reducing the loss of SO2 to :the stack. The liquor from the tank 48 goes to lthe recycling 'line of the first Venturi 42 where additional Mg(OH)2 is 'added in a controlled manner to give the i'inal desired combined SO2 contentfor the liquor.

The cooking acid is withdrawn from the strong acid storage tank 58 as needed in -the digester-IG. The cooking acid is readily maintained in the tank at the desired temperature and essentially free yof excess sulphurous acid. For example, :the total SO2 may be 5.04%, combined SO2 2.52 and free SO2 2.52.

In producing a chemical pulp from spruce, as `disclosed in isaid prior joint application, approximately 5 lb. of magnesia are required for producing the bisullphite needed for digestion of 100 lb. of wood chips. Approximately 3 to 4 lb. additional MgO per 100 1b. of Wood are required for neutralizing the liquor in the ldigester according to the presentinvention, ie. an increase of 60% or greater over that required for single stage cooking. Thus the recovery of MgO from the pulp residual liquor becomes of even greater importance with the present two-stage cooking process. Thus the digest-ion neutralization stage and the residual liquor recovery have a special inter-relationship making this simultaneous use of special importance.

This application is a continuation-in-part of my said prior joint application with George H. Tomlinson, Serial No. 625,164, filed Nov. 29, 1956, now U.S. Patent No. 3,046,182.

While in accordance with the provisions of the statutes I have illustrated and described herein a speciiic form of the invention now known to me, those skilled in the art will understand that changes may be made in the form of the -apparatus :disclosed without departing from the spirit of the invention covered by my claims, and that certain features of the invention may sometimes be used to advantage without a corresponding use of other features. In particular, the two-stage digesting process is readily carried out in continuous digesting equipment.

What is claimed is:

1. The method of preparing a chemical pulp from cellulosic raw material comprising impregnating the material with a magnesium bisulphite solution essentially free of excess sulphurous acid and initially in the range 2.5-5.0 pH, digesting the cellulosic material at a temperature between 160 and 200 C., for a predetermined digestion period while maintaining the pH of the solution as measured at room temperature at a value between 3.0 and 4.0 during the period when the major sulphonation of the l-ignin and dissolution from the cellulose takes place, and increasing the pH of the solution to -a maximum value in the range of 4.75-6.5 las measured at room temperature during the nal portion of the digestion period by the addition of an alkaline magnesium compound to the solution to form a deiibred pulp without mechanical refining.

2. Process for the recovery of cellulose from wood chips and the like by impregnating the wood chips with a magnesium bisulphite solution andcooking the impregnated wood chips to form a chemical pulp, characterized in that the chemical Vcomposition of the solution used for impregnating the chips corresponds approximately to the formula Mg(HSO3)2, i.e. is essentially free of excess sulphurous acid, the cooking temperature is maintained in the range between and 200 C., and the pH value of the solution las measured at room temperature is maintained at a value between 3.0 and 4.0 during the period when the major sulphonation of the lignin and disso'lution from the wood takes place, and maintaining the pH of the solution as measured at room temperature in the range 'of 6.0 `and 6.5 during the final portion of the digestion period by the addition of an alkaline magnesium compound rto the solution to form a deibred pulp without mechanical refining.

3. The method of preparing a chemical pulp from cellulosic raw material comprising impregnating the material with -a magnesium bisulphite solution essentially free of excess sulphurous acid and initially in the range `2.5-5.0 pH, digesting the cellulosic material at a temperature between 160 and 200 C., for a predetermined digestion period varying between approximately ten minutes and four hours, the lower the digestingtemperature -the longer the digestion time and vice versa, while maintaining the pH of the solution during a major portion of the digestion period between 3.0 and 4.0 as measured at room temperature, and during Ithe nal portion of the digestion period increasing the pH to a maximum value in the range of 4.75-6.5 as measured 'at room temperature by the addition of magnesium hydroxide to the solution, to form a deiiored pulp without mechanicai reiining.

4. The method of preparing a chemical pulp from cellulosic raw material comprising impregnating the material with a magnesium bisulphite solution essentially free of excess sulphurous acid and initially in the range 3.5-5.0 pH, and digesting the cellulosic material at a temperature between 160 and 200 C., for a predetermined digestion period while maintaining the piti or" the solution during a major portion of the digestion period between 3.0 and 4.0 as measured at room temperature by regulating the digesting pressure to retain sulphurous acid iiberated during the digestion, maintaining the pH of the solution as measured at room temperature in the range of 6.0 and 6.5 by the addition of magnesium hydroxide to the solution lduring the iinal portion of the digestion period, and increasing the digesting temperature during the period in which the increased pH is maintained, to form a deiibred pulp without mechanical reiining.

5. A cyclic process of preparing a chemical pulp from a cellulosic raw material which comprises impregnating the material with a cooking liquor consisting of a magnesium bisulphite solution essentially free of excess sulphurous acid and having a pH of 2.5 to 5.0, rapidly heating the material to a cooking temperature in the range of 160-200 C., holding the digester at said cooking temperature for a predetermined period While maintaining the pH of the solution as measured at room temperature at a value between 3.0 and 4.0 during the period when the major sulphonation of the lignin and dissolution from the cellulose takes place7 increasing the pH of the solution in the digester to a maximum value in the range oi 4.75-6.5 as measured at room temperature during the final portion of the digestion period by the addition of an alkaline magnesium compound to the solution, separating the residual liquor from the pulp, concentrating the residual liquor to a predetermined solids concentration suitable for self-sustaining combustion, burning the concentrated liquor with the production of heat, magnesium oxide and gaseous products including sulphur dioxide, forming a slurry of magnesium hydroxide, contacting the gaseous 1 1 products with the magnesium hydroxide slurry to form an aqueous solution of magnesium bisulphite essentially free of excess sulphurous acid, and recycling the reformed liquor essentially free of excess sulphurous acid.

6. A cyclic process of preparing a chemical pulp from a cellulosic raw material which comprises steaming chips of the cellulosic material in a digester to displace air,

impregnating the chips with a cooking liquor consisting of a magnesium bisulphite solution essentially free of excess sulphurous acid and having a pH of 2.5 to 5.0, rapidly heating the chips to a cooking temperature in the range of i60-200 C., holding the digester at said cooking temperature for a predetermined period While maintaining the digester pressure Iat a value slightly greater than the vapor pressure of pure Water at the digester temperature and maintaining the pH of the solution as measured at room temperature at a value between 3.0- and 4.0 during the period when the major sulphonation of the lignin and dissolution from the cellulose takes place,

maintaining the pH of the solution as measured at room temperature in the range of 6.0 `and 6.5 by lthe addition of magnesium hydroxide to the solution during the nal portionof the digestion period, separating the residual liquor from the pulp, concentrating the residual liquor to a predetermined solids concentration suitable for self-sustaining combustion, burning the concentrated liquor with the production of heat, magnesium oxide and gaseous products including sulphur dioxide, forming a `liquor to the digester While maintaining the reformed excess sulphurous acid and initially in the range 2.5-5.0

-pH, and the impregnated cellulosic material digested'at a temperature above 150 C. for a predetermined digestion period While the pH of the solution as measured at room temperature is maintained at a value'between 3.0 and 4.() during the period when the major sulphonation of the lignin and dissolution from the cellulose takes place, characterized by increasing the pH of the solution to a maximum value in the range of 4.75-"65 as 'measured at room temperature during the final portion of the digestion period by the addition of an alkaline magnesium compound to the solution.

8. A method of preparing a chemical pulp from cellulosic raW material according to claim 7 in which the increase in the pH of the solution is eiected by the addition of magnesium hydroxide to the solution. l

9. A method of preparing a chemical pulp from cellulosic raw material according to claim 8 in which the digesting temperature is increased during the digestion period in which the increased pH is maintained.`

10. A method of preparing a chemical pulp from cellulosic raw material in which the cel-lulosic material is digested with a magnesium bisulphite solution essentially free of excess sulphurous acid and at a pH value when measured at room temperature in the range of 3.0 to 4.0 during the period when the major sulphonation of the lignin and dissolution from the cellulose takes place, characterized by maintaining the ApH of the solution as measured at room temperature in the rangey of 4.75-6.5 during the iinal portionrof the digestion period by the addition of an alkaline magnesium compound to the solution.

References Cited in the le of this patent UNITED STATES PATENTS 1,864,620 Richter June 28, 1932 2,019,598 Dreyfus Nov. 5, 1935 2,118,074 Dreyfus May 24, 1938 2,385,955 Tomlinson /Oct. 2, 1945 2,749,241 Marpillero June S, 1956 

1. THE METHOD OF PREPARING A CHEMICAL PULP FROM CELLULOSIC RAW MATERIAL COMPRISING IMPREGNATING THE MATERIAL WITH A MAGNESIUM BISULPHITE SOLUTION ESSENTIALLY FREE OF EXCESS SULPHUROUS ACID AND INITIALLY IN THE RANGE 2.5-5.0 PH, DIGESTING THE CELLULOSIC MATERIAL AT A TEMPERATURE BETWEEN 160 AND 200*C., FOR A PREDETERMINED DIGESTION PERIOD WHILE MAINTAINING THE PH OF THE SOLUTION AS MEASURED AT ROOM TEMPERATURE AT A VALUE BETWEEN 3.0 AND 4.0 DURING THE PERIOD WHEN THE MAJOR SULPHONATION OF THE LIGNIN AND DISSOLUTION FROM THE CELLULOSE TAKES PLACE, AND INCREASING THE PH OF THE SOLUTION TO A MAXIMUM VALUE IN THE RANGE OF 4.75-6.5 AS MEASURED AT ROOM TEMPERATURE DURING THE FINAL PORTION OF THE DIGESTION PERIOD BY THE ADDITION OF AN ALKALINE MAGNESIUM COMPOUND TO THE SOLUTION TO FORM A DEFIBRED PULP WITHOUT MECHANICAL REFINING. 